Internet Engineering Task Force (IETF) A. Begen
Request for Comments: 6015 Cisco
Category: Standards Track October 2010
ISSN: 2070-1721
RTP Payload Format for 1-D Interleaved ParityForward Error Correction (FEC)
Abstract
This document defines a new RTP payload format for the Forward Error
Correction (FEC) that is generated by the 1-D interleaved parity code
from a source media encapsulated in RTP. The 1-D interleaved parity
code is a systematic code, where a number of repair symbols are
generated from a set of source symbols and sent in a repair flow
separate from the source flow that carries the source symbols. The
1-D interleaved parity code offers a good protection against bursty
packet losses at a cost of reasonable complexity. The new payload
format defined in this document should only be used (with some
exceptions) as a part of the Digital Video Broadcasting-IPTV (DVB-
IPTV) Application-layer FEC specification.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6015.
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RFC 6015 RTP Payload Format for Interleaved FEC October 2010
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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RFC 6015 RTP Payload Format for Interleaved FEC October 20101. Introduction
This document extends the Forward Error Correction (FEC) header
defined in [RFC2733] and uses this new FEC header for the FEC that is
generated by the 1-D interleaved parity code from a source media
encapsulated in RTP [RFC3550]. The resulting new RTP payload format
is registered by this document.
The type of the source media protected by the 1-D interleaved parity
code can be audio, video, text, or application. The FEC data are
generated according to the media type parameters that are
communicated through out-of-band means. The associations/
relationships between the source and repair flows are also
communicated through out-of-band means.
The 1-D interleaved parity FEC uses the exclusive OR (XOR) operation
to generate the repair symbols. In a nutshell, the following steps
take place:
1. The sender determines a set of source packets to be protected
together based on the media type parameters.
2. The sender applies the XOR operation on the source symbols to
generate the required number of repair symbols.
3. The sender packetizes the repair symbols and sends the repair
packet(s) along with the source packets to the receiver(s) (in
different flows). The repair packets may be sent proactively or
on demand.
Note that the source and repair packets belong to different source
and repair flows, and the sender needs to provide a way for the
receivers to demultiplex them, even in the case in which they are
sent in the same transport flow (i.e., same source/destination
address/port with UDP). This is required to offer backward
compatibility (see Section 4). At the receiver side, if all of the
source packets are successfully received, there is no need for FEC
recovery and the repair packets are discarded. However, if there are
missing source packets, the repair packets can be used to recover the
missing information. Block diagrams for the systematic parity FEC
encoder and decoder are sketched in Figures 1 and 2, respectively.
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RFC 6015 RTP Payload Format for Interleaved FEC October 20101.2. Overhead Computation
The overhead is defined as the ratio of the number of bytes that
belong to the repair packets to the number of bytes that belong to
the protected source packets.
Assuming that each repair packet carries an equal number of bytes
carried by a source packet and ignoring the size of the FEC header,
we can compute the overhead as follows:
Overhead = 1/D
where D is the number of rows in the source block.
1.3. Relation to Existing Specifications
This section discusses the relation of the current specification to
other existing specifications.
1.3.1. RFCs 2733 and 3009
The current specification extends the FEC header defined in [RFC2733]
and registers a new RTP payload format. This new payload format is
not backward compatible with the payload format that was registered
by [RFC3009].
1.3.2. SMPTE 2022-1
In 2007, the Society of Motion Picture and Television Engineers
(SMPTE) - Technology Committee N26 on File Management and Networking
Technology - decided to revise the Pro-MPEG Code of Practice (CoP) #3
Release 2 specification (initially produced by the Pro-MPEG Forum in
2004), which discussed several aspects of the transmission of MPEG-2
transport streams over IP networks. The new SMPTE specification is
referred to as [SMPTE2022-1].
The Pro-MPEG CoP #3 Release 2 document was originally based on
[RFC2733]. SMPTE revised the document by extending the FEC header
proposed in [RFC2733] (by setting the E bit). This extended header
offers some improvements.
For example, instead of utilizing the bitmap field used in [RFC2733],
[SMPTE2022-1] introduces separate fields to convey the number of rows
(D) and columns (L) of the source block as well as the type of the
repair packet (i.e., whether the repair packet is an interleaved FEC
packet computed over a column or a non-interleaved FEC packet
computed over a row). These fields, plus the base sequence number,
allow the receiver side to establish associations between the source
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RFC 6015 RTP Payload Format for Interleaved FEC October 2010
and repair packets. Note that although the bitmap field is not
utilized, the FEC header of [SMPTE2022-1] inherently carries over the
bitmap field from [RFC2733].
On the other hand, some parts of [SMPTE2022-1] are not in compliance
with RTP [RFC3550]. For example, [SMPTE2022-1] sets the
Synchronization Source (SSRC) field to zero and does not use the
timestamp field in the RTP headers of the repair packets (receivers
ignore the timestamps of the repair packets). Furthermore,
[SMPTE2022-1] also sets the CSRC Count (CC) field in the RTP header
to zero and does not allow any Contributing Source (CSRC) entry in
the RTP header.
The current document adopts the extended FEC header of [SMPTE2022-1]
and registers a new RTP payload format. At the same time, this
document fixes the parts of [SMPTE2022-1] that are not compliant with
RTP [RFC3550], except the one discussed below.
The baseline header format first proposed in [RFC2733] does not have
fields to protect the P and X bits and the CC fields of the source
packets associated with a repair packet. Rather, the P bit, X bit,
and CC field in the RTP header of the repair packet are used to
protect those bits and fields. This, however, may sometimes result
in failures when doing the RTP header validity checks as specified in
[RFC3550]. While this behavior has been fixed in [RFC5109], which
obsoleted [RFC2733], the RTP payload format defined in this document
still allows this behavior for legacy purposes. Implementations
following this specification must be aware of this potential issue
when RTP header validity checks are applied.
1.3.3. ETSI TS 102 034
In 2009, the Digital Video Broadcasting (DVB) consortium published a
technical specification [ETSI-TS-102-034] through the European
Telecommunications Standards Institute (ETSI). This specification
covers several areas related to the transmission of MPEG-2 transport
stream-based services over IP networks.
Annex E of [ETSI-TS-102-034] defines an optional protocol for
Application-layer FEC (AL-FEC) protection of streaming media for
DVB-IP services carried over RTP [RFC3550] transport. The DVB-IPTV
AL-FEC protocol uses two layers for protection: a base layer that is
produced by a packet-based interleaved parity code, and an
enhancement layer that is produced by a Raptor code [DVB-AL-FEC].
While the use of the enhancement layer is optional, the use of the
base layer is mandatory wherever AL-FEC is used. The DVB-IPTV AL-FEC
protocol is also described in [DVB-AL-FEC].
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RFC 6015 RTP Payload Format for Interleaved FEC October 2010
The interleaved parity code that is used in the base layer is a
subset of [SMPTE2022-1]. In particular, the AL-FEC base layer uses
only the 1-D interleaved FEC protection from [SMPTE2022-1]. The new
RTP payload format that is defined and registered in this document
(with some exceptions listed in [DVB-AL-FEC]) is used as the AL-FEC
base layer.
1.4. Scope of the Payload Format
The payload format specified in this document must only be used in
legacy applications where the limitations explained in Section 1.3.2
are known not to impact any system components or other RTP elements.
Whenever possible, a payload format that is fully compliant with
[RFC3550], such as [RFC5109] or other newer payload formats, must be
used.
2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Definitions, Notations, and Abbreviations
The definitions and notations commonly used in this document are
summarized in this section.
3.1. Definitions
This document uses the following definitions:
Source Flow: The packet flow(s) carrying the source data to which FEC
protection is to be applied.
Repair Flow: The packet flow(s) carrying the repair data.
Symbol: A unit of data. Its size, in bytes, is referred to as the
symbol size.
Source Symbol: The smallest unit of data used during the encoding
process.
Repair Symbol: Repair symbols are generated from the source symbols.
Source Packet: Data packets that contain only source symbols.
Repair Packet: Data packets that contain only repair symbols.
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RFC 6015 RTP Payload Format for Interleaved FEC October 2010
Source Block: A block of source symbols that are considered together
in the encoding process.
3.2. Notations
o L: Number of columns of the source block.
o D: Number of rows of the source block.
4. Packet Formats
This section defines the formats of the source and repair packets.
4.1. Source Packets
The source packets need to contain information that identifies the
source block and the position within the source block occupied by the
packet. Since the source packets that are carried within an RTP
stream already contain unique sequence numbers in their RTP headers
[RFC3550], we can identify the source packets in a straightforward
manner, and there is no need to append additional field(s). The
primary advantage of not modifying the source packets in any way is
that it provides backward compatibility for the receivers that do not
support FEC at all. In multicast scenarios, this backward
compatibility becomes quite useful as it allows the non-FEC-capable
and FEC-capable receivers to receive and interpret the same source
packets sent in the same multicast session.
4.2. Repair Packets
The repair packets MUST contain information that identifies the
source block to which they pertain and the relationship between the
contained repair symbols and the original source block. For this
purpose, we use the RTP header of the repair packets as well as
another header within the RTP payload, which we refer to as the FEC
header, as shown in Figure 6.
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RFC 6015 RTP Payload Format for Interleaved FEC October 2010
+------------------------------+
| IP Header |
+------------------------------+
| Transport Header |
+------------------------------+
| RTP Header | __
+------------------------------+ |
| FEC Header | \
+------------------------------+ > RTP Payload
| Repair Symbols | /
+------------------------------+ __|
Figure 6: Format of repair packets
The RTP header is formatted according to [RFC3550] with some further
clarifications listed below:
o Version: The version field is set to 2.
o Padding (P) Bit: This bit is equal to the XOR sum of the
corresponding P bits from the RTP headers of the source packets
protected by this repair packet. However, padding octets are
never present in a repair packet, independent of the value of the
P bit.
o Extension (X) Bit: This bit is equal to the XOR sum of the
corresponding X bits from the RTP headers of the source packets
protected by this repair packet. However, an RTP header extension
is never present in a repair packet, independent of the value of
the X bit.
o CSRC Count (CC): This field is equal to the XOR sum of the
corresponding CC values from the RTP headers of the source packets
protected by this repair packet. However, a CSRC list is never
present in a repair packet, independent of the value of the CC
field.
o Marker (M) Bit: This bit is equal to the XOR sum of the
corresponding M bits from the RTP headers of the source packets
protected by this repair packet.
o Payload Type: The (dynamic) payload type for the repair packets is
determined through out-of-band means. Note that this document
registers a new payload format for the repair packets (refer to
Section 5 for details). According to [RFC3550], an RTP receiver
that cannot recognize a payload type must discard it. This action
provides backward compatibility. The FEC mechanisms can then be
used in a multicast group with mixed FEC-capable and non-FEC-
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RFC 6015 RTP Payload Format for Interleaved FEC October 2010
capable receivers. If a non-FEC-capable receiver receives a
repair packet, it will not recognize the payload type, and hence,
discards the repair packet.
o Sequence Number (SN): The sequence number has the standard
definition. It MUST be one higher than the sequence number in the
previously transmitted repair packet. The initial value of the
sequence number SHOULD be random (unpredictable) [RFC3550].
o Timestamp (TS): The timestamp SHALL be set to a time corresponding
to the repair packet's transmission time. Note that the timestamp
value has no use in the actual FEC protection process and is
usually useful for jitter calculations.
o Synchronization Source (SSRC): The SSRC value SHALL be randomly
assigned as suggested by [RFC3550]. This allows the sender to
multiplex the source and repair flows on the same port or
multiplex multiple repair flows on a single port. The repair
flows SHOULD use the RTP Control Protocol (RTCP) CNAME field to
associate themselves with the source flow.
In some networks, the RTP Source (which produces the source
packets) and the FEC Source (which generates the repair packets
from the source packets) may not be the same host. In such
scenarios, using the same CNAME for the source and repair flows
means that the RTP Source and the FEC Source MUST share the same
CNAME (for this specific source-repair flow association). A
common CNAME may be produced based on an algorithm that is known
both to the RTP and FEC Source. This usage is compliant with
[RFC3550].
Note that due to the randomness of the SSRC assignments, there is
a possibility of SSRC collision. In such cases, the collisions
MUST be resolved as described in [RFC3550].
Note that the P bit, X bit, CC field, and M bit of the source packets
are protected by the corresponding bits/fields in the RTP header of
the repair packet. On the other hand, the payload of a repair packet
protects the concatenation of (if present) the CSRC list, RTP
extension, payload, and padding of the source RTP packets associated
with this repair packet.
The FEC header is 16 octets. The format of the FEC header is shown
in Figure 7.
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RFC 6015 RTP Payload Format for Interleaved FEC October 2010
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SN base low | Length recovery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| PT recovery | Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS recovery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|N|D|Type |Index| Offset | NA | SN base ext |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Format of the FEC header
The FEC header consists of the following fields:
o The SN base low field is used to indicate the lowest sequence
number, taking wraparound into account, of those source packets
protected by this repair packet.
o The Length recovery field is used to determine the length of any
recovered packets.
o The E bit is the extension flag introduced in [RFC2733] and used
to extend the [RFC2733] FEC header.
o The PT recovery field is used to determine the payload type of the
recovered packets.
o The Mask field is not used.
o The TS recovery field is used to determine the timestamp of the
recovered packets.
o The N bit is the extension flag that is reserved for future use.
o The D bit is not used.
o The Type field indicates the type of the error-correcting code
used. This document defines only one error-correcting code.
o The Index field is not used.
o The Offset and NA fields are used to indicate the number of
columns (L) and rows (D) of the source block, respectively.
o The SN base ext field is not used.
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RFC 6015 RTP Payload Format for Interleaved FEC October 2010
The details on setting the fields in the FEC header are provided in
Section 6.2.
It should be noted that a Mask-based approach (similar to the one
specified in [RFC2733]) may not be very efficient to indicate which
source packets in the current source block are associated with a
given repair packet. In particular, for the applications that would
like to use large source block sizes, the size of the Mask that is
required to describe the source-repair packet associations may be
prohibitively large. Instead, a systematized approach is inherently
more efficient.
5. Payload Format Parameters
This section provides the media subtype registration for the 1-D
interleaved parity FEC. The parameters that are required to
configure the FEC encoding and decoding operations are also defined
in this section.
5.1. Media Type Registration
This registration is done using the template defined in [RFC4288] and
following the guidance provided in [RFC4855].
5.1.1. Registration of audio/1d-interleaved-parityfec
Type name: audio
Subtype name: 1d-interleaved-parityfec
Required parameters:
o rate: The RTP timestamp (clock) rate in Hz. The (integer) rate
SHALL be larger than 1000 to provide sufficient resolution to RTCP
operations. However, it is RECOMMENDED to select the rate that
matches the rate of the protected source RTP stream.
o L: Number of columns of the source block. L is a positive integer
that is less than or equal to 255.
o D: Number of rows of the source block. D is a positive integer
that is less than or equal to 255.
o repair-window: The time that spans the FEC block (i.e., source
packets and the corresponding repair packets). An FEC encoder
processes a block of source packets and generates a number of
repair packets, which are then transmitted within a certain
duration not larger than the value of the repair window. At the
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RFC 6015 RTP Payload Format for Interleaved FEC October 2010
receiver side, the FEC decoder should wait at least for the
duration of the repair window after getting the first packet in an
FEC block to allow all the repair packets to arrive (the waiting
time can be adjusted if there are missing packets at the beginning
of the FEC block). The FEC decoder can start decoding the already
received packets sooner; however, it SHOULD NOT register an FEC
decoding failure until it waits at least for the repair-window
duration. The size of the repair window is specified in
microseconds.
Optional parameters: None.
Encoding considerations: This media type is framed (see Section 4.8
in the template document [RFC4288]) and contains binary data.
Security considerations: See Section 9 of [RFC6015].
Interoperability considerations: None.
Published specification: [RFC6015].
Applications that use this media type: Multimedia applications that
want to improve resiliency against packet loss by sending redundant
data in addition to the source media.
Additional information: None.
Person & email address to contact for further information: Ali Begen
<abegen@cisco.com> and the IETF Audio/Video Transport Working Group.
Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP [RFC3550].
Author: Ali Begen <abegen@cisco.com>.
Change controller: IETF Audio/Video Transport Working Group delegated
from the IESG.
5.1.2. Registration of video/1d-interleaved-parityfec
Type name: video
Subtype name: 1d-interleaved-parityfec
Required parameters:
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RFC 6015 RTP Payload Format for Interleaved FEC October 2010
o rate: The RTP timestamp (clock) rate in Hz. The (integer) rate
SHALL be larger than 1000 to provide sufficient resolution to RTCP
operations. However, it is RECOMMENDED to select the rate that
matches the rate of the protected source RTP stream.
o L: Number of columns of the source block. L is a positive integer
that is less than or equal to 255.
o D: Number of rows of the source block. D is a positive integer
that is less than or equal to 255.
o repair-window: The time that spans the FEC block (i.e., source
packets and the corresponding repair packets). An FEC encoder
processes a block of source packets and generates a number of
repair packets, which are then transmitted within a certain
duration not larger than the value of the repair window. At the
receiver side, the FEC decoder should wait at least for the
duration of the repair window after getting the first packet in an
FEC block to allow all the repair packets to arrive (the waiting
time can be adjusted if there are missing packets at the beginning
of the FEC block). The FEC decoder can start decoding the already
received packets sooner; however, it SHOULD NOT register an FEC
decoding failure until it waits at least for the repair-window
duration. The size of the repair window is specified in
microseconds.
Optional parameters: None.
Encoding considerations: This media type is framed (see Section 4.8
in the template document [RFC4288]) and contains binary data.
Security considerations: See Section 9 of [RFC6015].
Interoperability considerations: None.
Published specification: [RFC6015].
Applications that use this media type: Multimedia applications that
want to improve resiliency against packet loss by sending redundant
data in addition to the source media.
Additional information: None.
Person & email address to contact for further information: Ali Begen
<abegen@cisco.com> and the IETF Audio/Video Transport Working Group.
Intended usage: COMMON.
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RFC 6015 RTP Payload Format for Interleaved FEC October 2010
Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP [RFC3550].
Author: Ali Begen <abegen@cisco.com>.
Change controller: IETF Audio/Video Transport Working Group delegated
from the IESG.
5.1.3. Registration of text/1d-interleaved-parityfec
Type name: text
Subtype name: 1d-interleaved-parityfec
Required parameters:
o rate: The RTP timestamp (clock) rate in Hz. The (integer) rate
SHALL be larger than 1000 to provide sufficient resolution to RTCP
operations. However, it is RECOMMENDED to select the rate that
matches the rate of the protected source RTP stream.
o L: Number of columns of the source block. L is a positive integer
that is less than or equal to 255.
o D: Number of rows of the source block. D is a positive integer
that is less than or equal to 255.
o repair-window: The time that spans the FEC block (i.e., source
packets and the corresponding repair packets). An FEC encoder
processes a block of source packets and generates a number of
repair packets, which are then transmitted within a certain
duration not larger than the value of the repair window. At the
receiver side, the FEC decoder should wait at least for the
duration of the repair window after getting the first packet in an
FEC block to allow all the repair packets to arrive (the waiting
time can be adjusted if there are missing packets at the beginning
of the FEC block). The FEC decoder can start decoding the already
received packets sooner; however, it SHOULD NOT register an FEC
decoding failure until it waits at least for the repair-window
duration. The size of the repair window is specified in
microseconds.
Optional parameters: None.
Encoding considerations: This media type is framed (see Section 4.8
in the template document [RFC4288]) and contains binary data.
Security considerations: See Section 9 of [RFC6015].
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RFC 6015 RTP Payload Format for Interleaved FEC October 2010
Interoperability considerations: None.
Published specification: [RFC6015].
Applications that use this media type: Multimedia applications that
want to improve resiliency against packet loss by sending redundant
data in addition to the source media.
Additional information: None.
Person & email address to contact for further information: Ali Begen
<abegen@cisco.com> and the IETF Audio/Video Transport Working Group.
Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP [RFC3550].
Author: Ali Begen <abegen@cisco.com>.
Change controller: IETF Audio/Video Transport Working Group delegated
from the IESG.
5.1.4. Registration of application/1d-interleaved-parityfec
Type name: application
Subtype name: 1d-interleaved-parityfec
Required parameters:
o rate: The RTP timestamp (clock) rate in Hz. The (integer) rate
SHALL be larger than 1000 to provide sufficient resolution to RTCP
operations. However, it is RECOMMENDED to select the rate that
matches the rate of the protected source RTP stream.
o L: Number of columns of the source block. L is a positive integer
that is less than or equal to 255.
o D: Number of rows of the source block. D is a positive integer
that is less than or equal to 255.
o repair-window: The time that spans the FEC block (i.e., source
packets and the corresponding repair packets). An FEC encoder
processes a block of source packets and generates a number of
repair packets, which are then transmitted within a certain
duration not larger than the value of the repair window. At the
receiver side, the FEC decoder should wait at least for the
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RFC 6015 RTP Payload Format for Interleaved FEC October 2010
duration of the repair window after getting the first packet in an
FEC block to allow all the repair packets to arrive (the waiting
time can be adjusted if there are missing packets at the beginning
of the FEC block). The FEC decoder can start decoding the already
received packets sooner; however, it SHOULD NOT register an FEC
decoding failure until it waits at least for the repair-window
duration. The size of the repair window is specified in
microseconds.
Optional parameters: None.
Encoding considerations: This media type is framed (see Section 4.8
in the template document [RFC4288]) and contains binary data.
Security considerations: See Section 9 of [RFC6015].
Interoperability considerations: None.
Published specification: [RFC6015].
Applications that use this media type: Multimedia applications that
want to improve resiliency against packet loss by sending redundant
data in addition to the source media.
Additional information: None.
Person & email address to contact for further information: Ali Begen
<abegen@cisco.com> and the IETF Audio/Video Transport Working Group.
Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP [RFC3550].
Author: Ali Begen <abegen@cisco.com>.
Change controller: IETF Audio/Video Transport Working Group delegated
from the IESG.
5.2. Mapping to SDP Parameters
Applications that use RTP transport commonly use Session Description
Protocol (SDP) [RFC4566] to describe their RTP sessions. The
information that is used to specify the media types in an RTP session
has specific mappings to the fields in an SDP description. In this
section, we provide these mappings for the media subtype registered
by this document ("1d-interleaved-parityfec"). Note that if an
application does not use SDP to describe the RTP sessions, an
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RFC 6015 RTP Payload Format for Interleaved FEC October 2010
appropriate mapping must be defined and used to specify the media
types and their parameters for the control/description protocol
employed by the application.
The mapping of the media type specification for "1d-interleaved-
parityfec" and its parameters in SDP is as follows:
o The media type (e.g., "application") goes into the "m=" line as
the media name.
o The media subtype ("1d-interleaved-parityfec") goes into the
"a=rtpmap" line as the encoding name. The RTP clock rate
parameter ("rate") also goes into the "a=rtpmap" line as the clock
rate.
o The remaining required payload-format-specific parameters go into
the "a=fmtp" line by copying them directly from the media type
string as a semicolon-separated list of parameter=value pairs.
SDP examples are provided in Section 7.
5.2.1. Offer-Answer Model Considerations
When offering 1-D interleaved parity FEC over RTP using SDP in an
Offer/Answer model [RFC3264], the following considerations apply:
o Each combination of the L and D parameters produces a different
FEC data and is not compatible with any other combination. A
sender application may desire to offer multiple offers with
different sets of L and D values as long as the parameter values
are valid. The receiver SHOULD normally choose the offer that has
a sufficient amount of interleaving. If multiple such offers
exist, the receiver may choose the offer that has the lowest
overhead or the one that requires the smallest amount of
buffering. The selection depends on the application requirements.
o The value for the repair-window parameter depends on the L and D
values and cannot be chosen arbitrarily. More specifically, L and
D values determine the lower limit for the repair-window size.
The upper limit of the repair-window size does not depend on the L
and D values.
o Although combinations with the same L and D values but with
different repair-window sizes produce the same FEC data, such
combinations are still considered different offers. The size of
the repair-window is related to the maximum delay between the
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transmission of a source packet and the associated repair packet.
This directly impacts the buffering requirement on the receiver
side, and the receiver must consider this when choosing an offer.
o There are no optional format parameters defined for this payload.
Any unknown option in the offer MUST be ignored and deleted from
the answer. If FEC is not desired by the receiver, it can be
deleted from the answer.
5.2.2. Declarative Considerations
In declarative usage, like SDP in the Real-time Streaming Protocol
(RTSP) [RFC2326] or the Session Announcement Protocol (SAP)
[RFC2974], the following considerations apply:
o The payload format configuration parameters are all declarative
and a participant MUST use the configuration that is provided for
the session.
o More than one configuration may be provided (if desired) by
declaring multiple RTP payload types. In that case, the receivers
should choose the repair flow that is best for them.
6. Protection and Recovery Procedures
This section provides a complete specification of the 1-D interleaved
parity code and its RTP payload format.
6.1. Overview
The following sections specify the steps involved in generating the
repair packets and reconstructing the missing source packets from the
repair packets.
6.2. Repair Packet Construction
The RTP header of a repair packet is formed based on the guidelines
given in Section 4.2.
The FEC header includes 16 octets. It is constructed by applying the
XOR operation on the bit strings that are generated from the
individual source packets protected by this particular repair packet.
The set of the source packets that are associated with a given repair
packet can be computed by the formula given in Section 6.3.1.
The bit string is formed for each source packet by concatenating the
following fields together in the order specified:
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o Padding bit (1 bit) (This is the most significant bit of the bit
string.)
o Extension bit (1 bit)
o CC field (4 bits)
o Marker bit (1 bit)
o PT field (7 bits)
o Timestamp (32 bits)
o Unsigned network-ordered 16-bit representation of the source
packet length in bytes minus 12 (for the fixed RTP header), i.e.,
the sum of the lengths of all the following if present: the CSRC
list, header extension, RTP payload, and RTP padding (16 bits).
o If CC is nonzero, the CSRC list (variable length)
o If X is 1, the header extension (variable length)
o Payload (variable length)
o Padding, if present (variable length)
Note that if the lengths of the source packets are not equal, each
shorter packet MUST be padded to the length of the longest packet by
adding octet(s) of 0 at the end. Due to this possible padding and
mandatory FEC header, a repair packet has a larger size than the
source packets it protects. This may cause problems if the resulting
repair packet size exceeds the Maximum Transmission Unit (MTU) size
of the path over which the repair flow is sent.
By applying the parity operation on the bit strings produced from the
source packets, we generate the FEC bit string. Some parts of the
RTP header and the FEC header of the repair packet are generated from
the FEC bit string as follows:
o The first (most significant) bit in the FEC bit string is written
into the Padding bit in the RTP header of the repair packet.
o The next bit in the FEC bit string is written into the Extension
bit in the RTP header of the repair packet.
o The next 4 bits of the FEC bit string are written into the CC
field in the RTP header of the repair packet.
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o The next bit of the FEC bit string is written into the Marker bit
in the RTP header of the repair packet.
o The next 7 bits of the FEC bit string are written into the PT
recovery field in the FEC header.
o The next 32 bits of the FEC bit string are written into the TS
recovery field in the FEC header.
o The next 16 bits are written into the Length recovery field in the
FEC header. This allows the FEC procedure to be applied even when
the lengths of the protected source packets are not identical.
o The remaining bits are set to be the payload of the repair packet.
The remaining parts of the FEC header are set as follows:
o The SN base low field MUST be set to the lowest sequence number,
taking wraparound into account, of those source packets protected
by this repair packet.
o The E bit MUST be set to 1 to extend the [RFC2733] FEC header.
o The Mask field SHALL be set to 0 and ignored by the receiver.
o The N bit SHALL be set to 0 and ignored by the receiver.
o The D bit SHALL be set to 0 and ignored by the receiver.
o The Type field MUST be set to 0 and ignored by the receiver.
o The Index field SHALL be set to 0 and ignored by the receiver.
o The Offset field MUST be set to the number of columns of the
source block (L).
o The NA field MUST be set to the number of rows of the source block
(D).
o The SN base ext field SHALL be set to 0 and ignored by the
receiver.
6.3. Source Packet Reconstruction
This section describes the recovery procedures that are required to
reconstruct the missing source packets. The recovery process has two
steps. In the first step, the FEC decoder determines which source
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and repair packets should be used in order to recover a missing
packet. In the second step, the decoder recovers the missing packet,
which consists of an RTP header and RTP payload.
In the following, we describe the RECOMMENDED algorithms for the
first and second steps. Based on the implementation, different
algorithms MAY be adopted. However, the end result MUST be identical
to the one produced by the algorithms described below.
6.3.1. Associating the Source and Repair Packets
The first step is to associate the source and repair packets. The SN
base low field in the FEC header shows the lowest sequence number of
the source packets that form the particular column. In addition, the
information of how many source packets are available in each column
and row is available from the media type parameters specified in the
SDP description. This set of information uniquely identifies all of
the source packets associated with a given repair packet.
Mathematically, for any received repair packet, p*, we can determine
the sequence numbers of the source packets that are protected by this
repair packet as follows:
p*_snb + i * L (modulo 65536)
where p*_snb denotes the value in the SN base low field of the FEC
header of the p*, L is the number of columns of the source block and
0 <= i < D
where D is the number of rows of the source block.
We denote the set of the source packets associated with repair packet
p* by set T(p*). Note that in a source block whose size is L columns
by D rows, set T includes D source packets. Recall that 1-D
interleaved FEC protection can fully recover the missing information
if there is only one source packet missing in set T. If the repair
packet that protects the source packets in set T is missing, or the
repair packet is available but two or more source packets are
missing, then missing source packets in set T cannot be recovered by
1-D interleaved FEC protection.
6.3.2. Recovering the RTP Header and Payload
For a given set T, the procedure for the recovery of the RTP header
of the missing packet, whose sequence number is denoted by SEQNUM, is
as follows:
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1. For each of the source packets that are successfully received in
set T, compute the bit string as described in Section 6.2.
2. For the repair packet associated with set T, compute the bit
string in the same fashion except use the PT recovery field
instead of the PT field and TS recovery field instead of the
Timestamp field, and set the CSRC list, header extension and
padding to null regardless of the values of the CC field, X bit,
and P bit.
3. If any of the bit strings generated from the source packets are
shorter than the bit string generated from the repair packet,
pad them to be the same length as the bit string generated from
the repair packet. For padding, the padding of octet 0 MUST be
added at the end of the bit string.
4. Calculate the recovered bit string as the XOR of the bit strings
generated from all source packets in set T and the FEC bit
string generated from the repair packet associated with set T.
5. Create a new packet with the standard 12-byte RTP header and no
payload.
6. Set the version of the new packet to 2.
7. Set the Padding bit in the new packet to the first bit in the
recovered bit string.
8. Set the Extension bit in the new packet to the next bit in the
recovered bit string.
9. Set the CC field to the next 4 bits in the recovered bit string.
10. Set the Marker bit in the new packet to the next bit in the
recovered bit string.
11. Set the Payload type in the new packet to the next 7 bits in the
recovered bit string.
12. Set the SN field in the new packet to SEQNUM.
13. Set the TS field in the new packet to the next 32 bits in the
recovered bit string.
14. Take the next 16 bits of the recovered bit string and set the
new variable Y to whatever unsigned integer this represents
(assuming network order). Convert Y to host order and then take
Y bytes from the recovered bit string and append them to the new
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packet. Y represents the length of the new packet in bytes
minus 12 (for the fixed RTP header), i.e., the sum of the
lengths of all the following if present: the CSRC list, header
extension, RTP payload, and RTP padding.
15. Set the SSRC of the new packet to the SSRC of the source RTP
stream.
This procedure completely recovers both the header and payload of an
RTP packet.
7. Session Description Protocol (SDP) Signaling
This section provides an SDP [RFC4566] example. The following
example uses the FEC grouping semantics [RFC5956].
In this example, we have one source video stream (mid:S1) and one FEC
repair stream (mid:R1). We form one FEC group with the "a=group:
FEC-FR S1 R1" line. The source and repair streams are sent to the
same port on different multicast groups. The repair window is set to
200 ms.
v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=Interleaved Parity FEC Example
t=0 0
a=group:FEC-FR S1 R1
m=video 30000 RTP/AVP 100
c=IN IP4 233.252.0.1/127
a=rtpmap:100 MP2T/90000
a=mid:S1
m=application 30000 RTP/AVP 110
c=IN IP4 233.252.0.2/127
a=rtpmap:110 1d-interleaved-parityfec/90000
a=fmtp:110 L=5; D=10; repair-window=200000
a=mid:R1
8. Congestion Control Considerations
FEC is an effective approach to provide applications with resiliency
against packet losses. However, in networks where the congestion is
a major contributor to the packet loss, the potential impacts of
using FEC SHOULD be considered carefully before injecting the repair
flows into the network. In particular, in bandwidth-limited
networks, FEC repair flows may consume most or all of the available
bandwidth and may consequently congest the network. In such cases,
the applications MUST NOT arbitrarily increase the amount of FEC
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protection since doing so may lead to a congestion collapse. If
desired, stronger FEC protection MAY be applied only after the source
rate has been reduced.
In a network-friendly implementation, an application SHOULD NOT send/
receive FEC repair flows if it knows that sending/receiving those FEC
repair flows would not help at all in recovering the missing packets.
Such a practice helps reduce the amount of wasted bandwidth. It is
RECOMMENDED that the amount of FEC protection is adjusted dynamically
based on the packet loss rate observed by the applications.
In multicast scenarios, it may be difficult to optimize the FEC
protection per receiver. If there is a large variation among the
levels of FEC protection needed by different receivers, it is
RECOMMENDED that the sender offers multiple repair flows with
different levels of FEC protection and the receivers join the
corresponding multicast sessions to receive the repair flow(s) that
is best for them.
9. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [RFC3550] and in any applicable RTP profile.
The main security considerations for the RTP packet carrying the RTP
payload format defined within this memo are confidentiality,
integrity, and source authenticity. Confidentiality is achieved by
encrypting the RTP payload. Altering the FEC packets can have a big
impact on the reconstruction operation. An attack that changes some
bits in the FEC packets can have a significant effect on the
calculation and the recovery of the source packets. For example,
changing the length recovery field can result in the recovery of a
packet that is too long. Depending on the application, it may be
helpful to perform a sanity check on the received source and FEC
packets before performing the recovery operation and to determine the
validity of the recovered packets before using them.
The integrity of the RTP packets is achieved through a suitable
cryptographic integrity protection mechanism. Such a cryptographic
system may also allow the authentication of the source of the
payload. A suitable security mechanism for this RTP payload format
should provide source authentication capable of determining if an RTP
packet is from a member of the RTP session.
Note that the appropriate mechanism to provide security to RTP and
payloads following this memo may vary. It is dependent on the
application, transport and signaling protocol employed. Therefore, a
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single mechanism is not sufficient, although if suitable, using the
Secure Real-time Transport Protocol (SRTP) [RFC3711] is RECOMMENDED.
Other mechanisms that may be used are IPsec [RFC4301] and Transport
Layer Security (TLS) [RFC5246]; other alternatives may exist.
If FEC protection is applied on already encrypted source packets,
there is no need for additional encryption. However, if the source
packets are encrypted after FEC protection is applied, the FEC
packets should be cryptographically as secure as the source packets.
Failure to provide an equal level of confidentiality, integrity, and
authentication to the FEC packets can compromise the source packets'
confidentiality, integrity or authentication since the FEC packets
are generated by applying XOR operation across the source packets.
10. IANA Considerations
New media subtypes are subject to IANA registration. For the
registration of the payload format and its parameters introduced in
this document, refer to Section 5.
11. Acknowledgments
A major part of this document is borrowed from [RFC2733], [RFC5109],
and [SMPTE2022-1]. Thus, the author would like to thank the authors
and editors of these earlier specifications. The author also thanks
Colin Perkins for his constructive suggestions for this document.
12. References12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP:
Session Description Protocol", RFC 4566,
July 2006.
[RFC5956] Begen, A., "Forward Error Correction Grouping
Semantics in Session Description Protocol",
RFC 5956, September 2010.
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